1. Field of the Invention
This invention relates to energetic compositions such as solid propellant, gas generant, and pyrotechnic compositions. More particularly, the invention is directed to compositions and methods for increasing the conductivity of energetic materials and reducing the possibility of premature ignition or explosion due to electrostatic discharge during manufacture, transportation, storage, and use.
2. Technology Background
The sensitivity to electrostatic discharge of energetic compositions, such as solid propellant, gas generant, and pyrotechnic compositions, is well known. Numerous sources of electrical discharge have been cited as possible causes of catastrophic explosion or premature ignition of rocket motors containing solid propellants. External sources include natural lightning, electromagnetic pulses, high power microwave energy, and the like. In addition, static electricity charges are normally present at the interfaces between the various phases in the propellant, insulation, liner and other parts of the rocket motor. Charging of surfaces may occur by surface-to-surface contact (triboelectric contact) and by the cracking or separation of the solid phase, as in fractoelectrification.
Sudden discharge of this electrostatic energy may result in an explosion of materials or generate sufficient heat to ignite the solid propellant. Such catastrophic events have the potential for causing harm to people and property.
One manufacturing operation which has been implicated as a cause of catastrophic discharge and premature propellant ignition is the core pulling operation, i.e., removal of the core molds from the solid propellant grain after the grain is cast. Other manufacturing operations have the potential for causing rapid electrostatic discharge. Such events may also occur during storage, transportation, and deployment of materials or rocket motor.
Composite solid propellants have a very complex microstructure consisting of a dense pack of particles embedded in a polymeric binder matrix. The particles typically comprise fuel, oxidizers, combustion control agents, and the like. The particles may have a wide variety of sizes, shapes and electrical properties. Electrostatic charges typically build up on the binder-filler interfaces, on the grain surface, as well as at the interfaces between other components of the propellant, e.g. at the interface between conductive particles such as aluminum powder and a nonconductive or less-conductive binder.
Certain propellant compositions have a greater conductivity than other compositions. For example, a propellant having a polar polymer may contain dissociated ionic species available for charge transport and would have relatively high conductivity. Such ionic species may be present from ammonium perchlorate dissolved in the polar binder. Electrostatic charges are readily dissipated and catastrophic discharge is unlikely with this type of propellant binder system.
In another propellant, the solid constituents are bound in a polybutadiene/acrylonitrile/acrylic acid terpolymer binder (PBAN). The binder polymer contains polar nitrile functional groups along its backbone. In this system, a quaternary benzyl alkyl ammonium chloride is added to the binder polymer during manufacturing. The polymer and the quaternary ammonium salt together provide a relatively high electrical conductivity.
Another commonly used binder system in solid rocket propellant compositions is hydroxy-terminated polybutadiene (HTPB). In contrast to the PEG and PBAN binder systems, HTPB binders are nonpolar and have an intrinsic high insulation value. Thus, HTPB-based propellants are more susceptible, under certain circumstances, to high charge build-up with the potential for catastrophic electrostatic discharge.
Some pyrotechnic compositions are comprised of solid particles embedded in polymers and are susceptible to electrostatic discharge as are solid propellants. Some pyrotechnic compositions are prepared without binders. The ingredients are either mixed dry or in an evaporative solvent. Dry mixing of pyrotechnic ingredients is particularly susceptible to electrostatic discharge. It is generally known that as air flows across a surface, charge buildup occurs. In dry mixing, there is a very large surface area, creating the potential for charge buildup and electrostatic discharge.
U.S. Pat. No. 3,765,334, granted Oct. 16, 1973 to Rentz et al. reports adding graphite to igniter compositions to prevent electrostatic charge build up. It is reported that at least 16 percent graphite is required to achieve adequate conductivity. Such amounts of graphite adversely affect performance of energetic materials.
U.S. Pat. Nos. 4,072,546 and 4,696,705 report including graphite fibers in solid propellant and gas generant compositions to provide structural reinforcement and burn rate control. However, it is known that even small amounts of graphite fibers markedly increase the processing viscosity of propellant compositions. Even slight increases in viscosity can detrimentally affect processing and propellant rheology.
From the foregoing, it will be appreciated that there is a need in the art for energetic compositions which have sufficient conductivity to reduce electrostatic discharge susceptibility, yet which are processible, retain energetic performance, and retain comparable ballistic, mechanical, and rheological properties. It would also be an advancement in the art to provide methods for reducing electrostatic discharge in energetic compositions.
Such energetic compositions and methods are disclosed and claimed herein.